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MC33067P Fiches technique(PDF) 7 Page - ON Semiconductor |
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MC33067P Fiches technique(HTML) 7 Page - ON Semiconductor |
7 / 16 page ![]() MC34067 MC33067 7 MOTOROLA ANALOG IC DEVICE DATA OPERATING DESCRIPTION Introduction As power supply designers have strived to increase power conversion efficiency and reduce passive component size, high frequency resonant mode power converters have emerged as attractive alternatives to conventional pulse–width modulated control. When compared to pulse–width modulated converters, resonant mode control offers several benefits including lower switching losses, higher efficiency, lower EMI emission, and smaller size. A new integrated circuit has been developed to support this trend in power supply design. The MC34067 Resonant Mode Controller is a high performance bipolar IC dedicated to variable frequency power control at frequencies exceeding 1.0 MHz. This integrated circuit provides the features and performance specifically for zero voltage switching resonant mode power supply applications. The primary purpose of the control chip is to provide a fixed off–time to the gates of external power MOSFETs at a repetition rate regulated by a feedback control loop. Additional features of the IC ensure that system startup and fault conditions are administered in a safe, controlled manner. A simplified block diagram of the IC is shown on the front page, which identifies the main functional blocks and the block–to–block interconnects. Figure 13 is a detailed functional diagram which accurately represents the internal circuitry. The various functions can be divided into two sections. The first section includes the primary control path which produces precise output pulses at the desired frequency. Included in this section are a variable frequency Oscillator, a One–Shot, a pulse Steering Flip–Flop, a pair of power MOSFET Drivers, and a wide bandwidth Error Amplifier. The second section provides several peripheral support functions including a voltage reference, undervoltage lockout, Soft–Start circuit, and a fault detector. Primary Control Path The output pulse width and repetition rate are regulated through the interaction of the variable frequency Oscillator, One–Shot timer and Error Amplifier. The Oscillator triggers the One–Shot which generates a pulse that is alternately steered to a pair of totem pole output drivers by a toggle Flip–Flop. The Error Amplifier monitors the output of the regulator and modulates the frequency of the Oscillator. High speed Schottky logic is used throughout the primary control channel to minimize delays and enhance high frequency characteristics. Oscillator The characteristics of the variable frequency Oscillator are crucial for precise controller performance at high operating frequencies. In addition to triggering the One–Shot timer and initiating the output deadtime, the oscillator also determines the initial voltage for the one–shot capacitor. The Oscillator is designed to operate at frequencies exceeding 1.0 MHz. The Error Amplifier can control the oscillator frequency over a 1000:1 frequency range, and both the minimum and maximum frequencies are easily and accurately programmed by the proper selection of external components. The functional diagram of the Oscillator and One–Shot timer is shown in Figure 14. The oscillator capacitor (COSC) is initially charged by transistor Q1. When COSC exceeds the 4.9 V upper threshold of the oscillator comparator, the base of Q1 is pulled low allowing COSC to discharge through the external resistor, (ROSC), and the oscillator control current, (IOSC). When the voltage on COSC falls below the comparator’s 3.6 V lower threshold, Q1 turns on and again charges COSC. COSC charges from 3.6 V to 5.1 V in less than 50 ns. The high slew rate of COSC and the propagation delay of the comparator make it difficult to control the peak voltage. This accuracy issue is overcome by clamping the base of Q1 through a diode to a voltage reference. The peak voltage of the oscillator waveform is thereby precisely set at 5.1 V. Figure 14. Oscillator and One–Shot Timer VOE Oscillator Control Current COSC RT CT ROSC IOSO 4.9V/3.6V 1 2 10 3 OSC Charge OSC RC One–Shot RC D1 Q1 IOSC Oscillator One–Shot 3.1V 4.9V/3.6V Vref Error Amp Output 6 VOE + + Error Amp Charge RVFO The frequency of the Oscillator is modulated by varying the current flowing out of the Oscillator Control Current (IOSC) pin. The IOSC pin is the output of a voltage regulator. The input of the voltage regulator is tied to the variable frequency oscillator. The discharge current of the Oscillator increases by increasing the current out of the IOSC pin. Resistor RVFO is used in conjunction with the Error Amp output to change the IOSC current. Maximum frequency occurs when the Error Amplifier output is at its low state with a saturation voltage of 0.1 V at 1.0 mA. The minimum oscillator frequency will result when the IOSC current is zero, and COSC is discharged through the external resistor (ROSC). This occurs when the Error Amplifier output is at its high state of 2.5 V. The minimum and maximum oscillator frequencies are programmed by the proper selection of resistor ROSC and RVFO. The minimum frequency is programmed by ROSC using Equation 1: COSC where tPD is the internal propagation delay. (1) R 1 ƒ tPD n C t 70 ns 0.348 – = = – OSC (min) (max) OSC 5.1 3.6 |
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